One solution for all-optical switching employs two MEMS devices each containing an array of tiltable micro mirrors, e.g., small mirrors, which can reflect light, which herein refers to any radiation in the wavelength of interest, whether or not in the visible spectrum. An optical path is established for light supplied from an input source, e.g., an optical fiber, to an output, e.g., an output fiber, by steering the light using a first micro mirror on the first optical MEMS device, the first micro mirror being associated with the input fiber, onto a second micro mirror on the second optical MEMS device which is associated with the output fiber. The second micro mirror then steers the light into the output fiber. Each fiber connected to the system is considered a port of the system, the input fibers being the input ports and the output fibers being the output ports.
There are various prior art methods of making such an array of tiltable micro mirrors. Typically the array is made in two parts. The first part includes the electrodes which control the tilt of the micro mirrors and some type of spacer which holds the second part offset from the electrodes. The second part includes the micro mirrors and their springs and any other supporting structure.
The spacers of one prior art mirror array are made from polyimide, which is photo-patternable type of plastic that is deposited on the substrate. Disadvantageously, such spacers are a) not flat at the top, b) are relatively soft, e.g., compared to silicon, c) must be hard baked at high temperatures, and d) the height of the resulting spacers is not uniform from device to device even when the same processing is employed.
In another prior art arrangement, the micro mirrors are manufactured from a silicon on insulator (SOI) wafer and then portions of the back of the wafer are etched to allow the mirror to move freely. The unetched portions serve as the spacers and keep the micro mirrors elevated with respect to the electrodes which are on a second wafer. See for example U.S. Pat. No. 6,201,631, which is incorporated by reference as if fully set forth herein. However, such micro mirrors are relatively fragile, and the height of the spacers is dictated by the thickness of the wafer on which the micro mirrors were formed, which is typically greater than 200 μm.
Yet another prior art method creates micro mirrors as suspended structures by depositing a thick sacrificial layer on a substrate, with appropriate patterning to make holes therethrough via etching. Then, a material to form the micro mirrors is conformally deposited on top of the sacrificial layer. The layer for the micro mirrors is patterned, and the micro mirrors are formed. A portion of the conformally deposited mirror material is then etched away to allow access to the sacrificial layer. Finally, the sacrificial layer is removed via etching. This process suffers from the fact that it takes a long time to grow the thick sacrificial layer, and the height of the suspended micro mirrors is typically limited by thickness of the sacrificial layer, so the height is often limited to no more than 5 μm.
A process similar to that used for making accelerometers as taught in “ISAAC:integrated silicon automotive accelerometer” by Leland ‘Chip’ Spangler and Christopher J. Kemp, published in Sensors and Actuators A 54 (1996), pages 523-529, which is incorporated by reference as if fully set forth herein, is unsuitable for making arrays of micro mirrors. This is because when using that process it is too hard to control the final thickness of the membrane that would be used for the mirror and other delicate structures, such as springs, which would also be formed from the same membrane.